Ionizing radiation detector

ABSTRACT

A CdZnTe (CZT) crystal provided with a native CdO dielectric coating to reduce surface leakage currents and thereby, improve the resolution of instruments incorporating detectors using CZT crystals is disclosed. A two step process is provided for forming the dielectric coating which includes etching the surface of a CZT crystal with a solution of the conventional bromine/methanol etch treatment, and passivating the CZT crystal surface with a solution of 10 w/o NH4F and 10 w/o H2O2 in water after attaching electrical contacts to the crystal surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of, and claims priority to, prior U.S.patent application Ser. No. 09/536,883 originally filed Mar. 28, 2000,now U.S. Pat. No. 6,524,966 and entitled “SURFACE TREATMENT ANDPROTECTION METHOD FOR CADMIUM ZINC TELLURIDE CRYSTALS,” which is itselfa continuation-in-part of, and claims priority from, U.S. patentapplication Ser. No. 09/118,691 titled “METHOD FOR SURFACE PASSIVATIONAND PROTECTION OF CADMIUM ZINC TELLURIDE CRYSTALS” filed Jul. 16, 1998by Mark Mescher, Ralph James, Tuviah Schlesinger, and Haim Hermon, andassigned to Sandia Corporation now U.S. Pat. No. 6,043,106. The entiredisclosure of both U.S. patent application Ser. Nos. 09/536,883 and09/118,691 is hereby incorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under governmentcontract no. DE-ACO04-94AL85000 awarded by the U.S. Department of Energyto Sandia Corporation. The Government has certain rights in theinvention, including a paid-up license and the right, in limitedcircumstances, to require the owner of any patent issuing in thisinvention to license others on reasonable terms.

BACKGROUND OF THE INVENTION Field of the Invention

This invention pertains generally to methods for improving theperformance of detectors for gamma-ray and x-ray spectrometers andimaging systems. More particularly, this invention pertains to a methodfor treating the surface of CdZnTe detector crystals to reduce leakagecurrents and increase spectral resolution.

Many of the commonly used radiation detectors employ Si(Li) or Gesemiconductor materials and thus operate most effectively at cryogenictemperatures and under very clean vacuum conditions. The need to operateSi(Li) or Ge-based detectors under these rigorous conditions posessignificant limitations on their use for applications where portabilityis desired.

The general requirements for room temperature operation ofsemiconducting materials as detectors for spectrometer applications arenumerous and in some case contradictory. In particular, in thisimportant that the chosen material exhibit a relatively large band gapenergy such that thermal generation of charge carriers is minimized.Conversely, however, a small band gap energy is necessary such that alarge number of electron-hole pairs is created for each absorbed quantumof ionizing radiation in order to maximize detector resolution. Inaddition, the material under consideration should have a relatively highaverage atomic number when used in gamma ray spectroscopy to increasethe gamma ray interaction probability. Lastly, a high charge carriermobility and long charge carrier lifetime are needed to ensure efficientcharge carrier extraction and minimal effects from position dependentcharge collection.

CdZnTe (CZT), and particularly Cd_(1−x)Zn_(x)Te (where x is less than orequal 0.5), is a wide bandgap ternary II-VI compound semiconductor that,because of its unique electronic properties, is desirable for use inroom temperature gamma-ray and x-ray radiation detection, spectroscopy,and medical imaging applications. However, the performance of gamma-rayand x-ray spectrometers which employ CZT detector crystals is oftenlimited by surface leakage currents which act as a source of noise thatreduces the ability of these spectrometers to spectrally resolve theunique radiological emissions from a wide variety of radioactiveisotopes. Thus, in order to improve the spectral resolution capabilityof devices based on CZT crystals it is desirable to decrease surfaceleakage currents and the attendant detrimental noise effects.

It is known, in the art, that for a semiconductor crystal to functioneffectively as a good detector material (i.e., minimizing surfaceleakage currents, thereby maximizing energy resolution) the crystalsurfaces must be properly “treated.” Generally speaking, this meanschemically etching of the surfaces to eliminate undesirable surfacefeatures. Currently, the generally accepted method for surface treatmentof CZT crystals is to chemically etch the crystal surfaces in a solutionof liquid bromine dissolved in methanol in order to provide a planarsurface prior to attachment of electrical contacts. These solutions, or“etchants,” are used because they reliably produce surfaces on CZTcrystals that are substantially planar and that have a low surfaceleakage current.

Applicant have also shown that it is possible to markedly reduce leakagecurrent, and therefore noise discrimination, in these crystals byincorporating a passivating layer on the surfaces of the crystal. Such asystem is described in co-pending U.S. patent application Ser. No.09/118,691 wherein a silicon nitride layer is sputtered onto a CZTcrystal surface as such a passivating layer.

However, there is a need to reduce the surface leakage current in CZTcrystals still further in order to improve spectral resolution. What isrequired is a method for surface treatment of CZT crystal that willeliminate or reduce surface leakage currents to a level that ispresently unattainable using prior art methods.

SUMMARY OF THE INVENTION

CdZnTe (CZT) crystals, particularly Cd_(1−x)Zn_(x)Te (where x is less orequal 0.5) crystals and preferably Cd_(0.9)Zn_(0.1)Te crystals, areuseful for fabrication of small, portable, room temperature radiationdetectors. In a first embodiment of the present invention a method istaught for treating a surface or surfaces of CZT crystals that providesa coating on the crystal surface which will reduce surface leakagecurrents to a previously unattainable level and thereby provide forimproved energy resolution in instruments incorporating CZT crystalsprocessed by this invention. A two step process is disclosed, whereinthe surface of a CZT crystal is etched the traditional bromine/methanoletch treatment (5 volume percent (v/o) bromine in methanol solution),and after attachment of electrical contacts the surface of the CZTcrystal is passivated, preferably by treatment with a aqueous solutionof ammonium fluoride and hydrogen peroxide.

A second embodiment of the present invention provides for a surfacetreatment of CZT crystals that reduces surface leakage currents andsimultaneously provides a hard-coat over-layer which preventsperformance decay over time due to exposure to moisture and other gasesin the working environment, thereby providing for improved energyresolution and reliability. This second embodiment includes the processof the first embodiment followed with an encapsulation techniquecomprising a low-temperature sputter-deposited silicon nitride overlayeror the application of a polymer seal layer known as HumiSeal® (HumiSeal®is a registered trademark of the Chase Corporation, Woodside, N.Y.).

It is an object of this invention therefore to provide a method forproducing a passivating dielectric layer on the surface of a CZTcrystal.

It is another object of this invention to provide a method for producinga thick, dense, coherent dielectric layer on the surface of a CZTcrystal.

Yet another object of this invention is to provide an enhanced oxidizingsolution treatment for producing a thick, dense, coherent dielectriclayer on the surface of a CZT crystal.

Still another object of this invention is to provide a method forpassivating a CZT crystal surface by immersing said surface in anoxidizing solution comprising hydrogen peroxide and ammonium fluoride inwater.

Yet another object of this invention is to provide a method forpassivating a CZT crystal surface by immersing said surface in anoxidizing solution comprising 10 w/o hydrogen peroxide and 10 w/oammonium fluoride in water.

Another object of this invention is to provide a method for reducingsurface current leakage in CZT crystals by factor of about 5 below thatachievable by conventional wet chemical methods.

Yet another object of this invention is to provide a CZT detectorincorporating a CZT crystal having a thick, dense, coherent native oxidedielectric layer covering all exposed surfaces

Yet another object of this invention is to provide a CZT crystal havinga dense, coherent oxide coating that consists essentially of cadmiumoxide.

Another object of this invention is to provide a CZT crystal having anoxide coating which at least greater than about 250 Å.

Still another object of this invention is to provide a high resistivityCZT crystal having a surface current leakage below about 0.01 nA.

A further object of this invention is the application of moisturebarrier over the passivating layer such as either a low-temperaturesputter-deposited overlayer or a moisture impermeable polymer layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows current/voltage curves for CZT crystals etched andpassivated using the conventional prior art method of bromine/methanoletching followed by hydrogen peroxide passivation.

FIG. 2A shows current/voltage curves for high resistivity CZT crystalsetched and passivated using the NH₄F/H₂O₂ surface treatment method ofthe instant invention.

FIG. 2A shows a magnified portion of FIG. 2A.

FIG. 3A shows current/voltage curves for low resistivity CZT crystalsetched and passivated using the NH₄F/H₂O₂ surface treatment method ofthe instant invention.

FIG. 3B shows a magnified portion of FIG. 3A.

FIG. 4 shows a conventional method for attaching electrode contacts toCZT crystals followed by a prior art method for sealing/passivating theexposed crystal surfaces to reduce surface current leakage.

FIG. 5A illustrates a photolithographic method for forming an electrodeon a CZT crystal surface followed by forming a passivating layer on theexposed crystal surfaces by using the method of the instant invention.

FIG. 5B shows a new method for attaching electrode contacts to CZTcrystals comprising forming an patterned oxide layer on a surface of aCZT crystal before forming the electrode followed by forming secondpattern between the oxide pattern into which the electrode is deposited.

FIG. 6 shows a photolithographic method for forming an electrode on aCZT crystal surface where the entire crystal is first passivated by themethod of the instant invention and where a pattern is subsequentlyetched through the oxide layer to expose a portion of the crystalsurface onto which an electrode contact is deposited.

FIG. 7 illustrates a simplified view of a magnetron sputtering system.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a novel and nonobvious method forpreparing a CZT crystal having a low value of leakage current which isunattainable with present surface treatment methods. The new processdisclosed herein comprises etching the surface of a CZT crystal,particularly a Cd_(1−x)Zn_(x)Te (where x is less or equal 0.5) crystal,and preferably a Cd_(0.9)Zn_(0.1)Te crystal, with a conventionalsolution of a 5 v/o bromine/methanol etch, applying electrical contacts,and finally passivating the surface of the etched CZT crystal,preferably with a aqueous solution of an ammonium salt such as ammoniumfluoride and hydrogen peroxide although, other halide ammonium salts,include NH₄F, NH₄Cl, NH₄Br, NH₄I, are also believed to be effective.

It is believed that surface leakage currents are dominated by twoeffects: 1) the multiplicity of grain boundaries in polycrystalline CZTat which electrically active tellurium precipitates, and 2) the presenceof a thin layer of non-stoichiometric material on the etched crystalsurface which has an electrical resistivity much lower than theunderlying bulk crystal material, the latter arising from the damagedone in cutting and polishing the crystal.

Unfortunately, chemical etchants do not act uniformly on all theindividual constituents of alloys such as CZT, and generally leavebehind regions of non-stoichiometric material. In the case of CZTcrystals etched in the conventional 5 v/o bromine/methanol solution,this non-stoichiometric material is believed to be a tellurium-richsurface layer having a conductivity that is substantially greater thanthe underlying stoichiometric CZT material. Since leakage current isproportional to electrical conductivity, it is desirable to remove thisnon-stoichiometric material from the crystal surface.

The surface of a CZT crystal could be improved, therefore, by one or acombination of the following three processes: 1) selectively removingthe electrically active species at the crystal surface, typically by anetching process; 2) chemically converting the active species to amolecular compound having dielectric properties; and 3) increasing thereaction rates of 2) such that the quantity of reaction product producedis enough to consume the surface layer damaged by cutting/polishing andthe non-stoichiometric layer produced by etching.

The present state of the art teaches forming an oxide layer on thesurface of the CZT material. Oxides are known to be insulating materialswith normal wide band gaps, high dielectric constant, high resistivity.Furthermore, oxides exhibit chemical stability in most non-acidicaqueous environments. Oxides have been grown from the native surface ofCZT by various methods. Some of those methods include: 1)wet chemicalproduction of reaction products at the CZT surface; 2) introduction ofthe CZT surface to an oxygen plasma; and, 3) bombardment of the CZTsurface with oxygen anions.

However, it has been found that the long-term stability of these oxidesdegrades with time. This may be due in part by the relatively thin layerproduced by these methods. For example, the wet chemical oxidizingprocess utilizing H₂O₂ provides an oxide layer ranging in thickness fromonly about 20 Å to 40 Å. It is believed that if the “as-grown” nativedielectric is not thick enough it will not consume enough of the CZTsurface to etch that surface. Thin oxide/dielectric films, therefore,primarily passivate through a process of changing electrically activedefects on the surface of a semiconductor crystal to molecular specieshaving favorable electrical characteristics. However, this processinherently results in a native oxide that is not uniformly distributedspatially which means that the density of the oxide film produced by theprocess is reduced, giving rise to a higher diffusion coefficient forreactive species through the layer. The higher diffusion coefficient ofthe dielectric layer means that reactive species, especially molecularoxygen and water vapor, can quickly diffuse through the layer givingrise to a continuous and on-going reaction at the interface of thedielectric layer and CZT. This diffusion-feed reaction through thedielectric layer creates instability in the oxide/dielectric layerinterface with time and/or applied bias ultimately leading to adegradation of device performance. More importantly, the density ofinterface states, i.e., positive and/or negative charges trapped at theinterface, arising as the result of structural or oxidation induceddefects, or due to the presence of metal impurities, will benon-uniformly distributed allowing for the dielectric strength of theoxide/dielectric layer to be compromised.

The present invention solves these two problems by a new process whichincludes etching the surface of a CZT crystal with a conventionalBr₂/methanol etchant to remove features having a high radius ofcurvature, followed immediately by applying electrical contacts to thesurface of the CZT crystal, (particularly electroless gold andpreferably gold electrodes deposited by thermally evaporation orsputtering process), and subsequently forming an oxide layer on thesurface of the CZT crystal having the electrical contacts. The finalstep of forming the oxide layer is performed preferably by immersing theCZT crystal surface in a ammonium fluoride/hydrogen peroxide aqueoussolution (10 weight percent (w/o) NH₄F and 10 weight percent (w/o) H₂O₂in water) for about ten (10) minutes. The density, thickness, mechanicalstability, and adherence properties of the NH₄F/H₂O₂ derived oxide layerhas been found to be far superior to the amorphous oxides formed byother methods.

It is believed that the hydrogen peroxide oxidizes the ammonium fluorideforming thereby hyponitrous acid (H₂N₂O₂), fluorine gas (F₂), anddi-fluorine oxide gas (F₂O). It is also believed that not all of theammonium fluoride is oxidized. This then leaves some remaining as freeammonium (NH^(+hd 4)) and fluoride (F⁻) ions in solution which arethought to aid in the formation of fluoride compounds of the elementalconstituents of CZT, (CdF₂, ZnF₂, TeF₄, and TeF₆) as the crystal surfaceis consumed. Finally, the formation of these halide compounds providesan essential, intermediate step in the formation of the dielectric oxidelayer since the direct oxidation of the CZT surface is known to bekinetically unfavorable. By attacking the crystal surface to formintermediate halide compounds at the CZT surface, the unoxidizedammonium fluoride has, in effect, replaced an unfavorable reaction, thatof direct oxidation of the pure CZT, with one that is favorable: thedecomposition of the halide compounds into their corresponding oxides,particularly cadmium.

The instant process, therefore, provides a primary dielectric layerwhich has been shown to be substantially a bulk layer of CdO. This is animportant result in that CdO is known to have a very high density andexhibits the best lattice match with CZT of the several constituentoxides and would be expected therefore to provide an inert and stableprotective layer. These findings have been verified through qualitativeanalysis utilizing Auger Electron Spectroscopy depth profilingmeasurements which reveals that the new passivation solution forms arobust dielectric layer on the surface of the CZT crystal about 300Å-450 Å thick and is comprised substantially of CdO with a thin outsidelayer of tellurium oxide (either TeO or TeO₂) believed to result fromthe oxidation of the out-diffusion of tellurium hexafluoride (TeF₆).Since the tellurium hexafluoride is known to exist as a gas at roomtemperature its presence therefore provides a possible mechanism for thepreferential depletion of Te from the etched CZT crystal surface and thesubsequent oxidization of cadmium to cadmium oxide (CdO): This isbelieved to allow for a countercurrent in-diffusion of O²⁻, providedprimarily through the in-diffusion of water molecules or left overhydrogen peroxide, driven by the concentration gradient produced by thedisplacement of Te²⁻ from the CZT crystal surface in the form oftellurium hexafluoride.

EXAMPLES

As can be seen from experimental results, summarized in TABLE 1 below,CZT crystals that are treated using the conventional prior art treatmenthave a smaller reduction in surface leakage current than those treatedwith the NH₄F/H₂O₂ treatment. In order to perform a set of passivationexperiments to quantify the surface leakage current between variouspassivation methods CZT crystal samples were selected, etched in a 5 v/obromine in methanol solution, rinsed in deionized water and dried in astream of nitrogen gas. The samples of both high and low resistivity CZTmaterial were tested. Circular gold electrodes were attached to frontand back faces of the etched crystals and a initial current-voltagemeasurement taken to establish a baseline. Crystals were then dippedrepeatedly into passivating solutions of either the conventional 10 w/ohydrogen peroxide solution, or the ammonium fluoride/hydrogen peroxidesolution of the present invention. Each dip immersion was timed for aninterval of 5 minutes after which the processed crystals were rinsed anddried as before, and subjected to subsequent current-voltagemeasurements to obtain a comparative measure of surface current leakageperformance against the baseline measurement.

The accompanying FIGURES illustrate the observed change in leakagecurrent with increasing bias voltage for various crystal types andtreatments.

Representative measurements are summarized in the TABLE below. Inparticular, a comparison of FIGS. 2 and 3 with FIG. 1 shows that thestep of growing a dielectric layer using the new oxidative processprovides a substantial improvement over the conventional peroxidepassivation method. As can be seen in the tabulated summary in TABLE 1this improvement amounts to decrease in leakage current in both the lowand high resistivity materials used in the experiment of several hundredpercent.

TABLE 1 Comparison of experimental results obtained with differentoxidant solutions, and with different semiconductor materialsPassivating Initial Final Factor of Initial Leakage Final Leakage %Reduction of Factor of Treatment Following Resistance [Ω] Resistance [Ω]Increase in R Current [nA] Current [nA] Leakage Current decrease in IEtching in Br/MeOH R_(i) R_(f) (R_(f)/R_(i)) I_(i) I_(i) (I_(i)/I_(f))H₂O₂ 2.81 × 10¹⁰ 3.78 × 10¹⁰ 1.35 4.96 4.1 19 1.21 High ResistanceMaterial (prior art) NH₄F/H₂O₂ 3.30 × 10¹⁰ 4.45 × 10¹³ 1349 1.48 1.59 ×10⁻³ 99.9 927 High Resistance Material NH₄F/H₂O₂ 2.06 × 10⁶ 9.02 × 10⁸437 3.2 × 10⁴ 142 99.6 225 Low resistance Material

FIG. 1 shows the leakage current vs. voltage curves for CZT crystalstreated by the prior art method of etching the crystal in a 5 v/osolution of bromine in methanol for 2 minutes followed by passivatingthe crystal in hydrogen peroxide. These measurements where made byattaching platinum leads, having a diameter of about 0.01 mm, usingAquadag (graphite suspension in water) to gold contacts deposited ontothe CZT surface by thermal evaporation, following the step of etching.The entire assembly was then covered with a protective coating, such asHumiSeal® supplied by the HumiSeal Division of the Chase Corporation.The current-voltage measurements shown in FIG. 1 were made at roomtemperature using, by way of example, a Keithley Model 617 programmableelectrometer in conjunction with a Bertan high voltage power supply.

While the conventional step of etching provided herein produces CZTdetector crystals having lower leakage currents, the inventors havefound that it is possible to further reduce the leakage current, andthus the noise level, and improve the sensitivity and spectralresolution of radiation detectors employing CZT crystals by passivatingthe surface of an etched CZT crystal; particularly with a solution ofammonium fluoride and hydrogen peroxide in water and preferably with asolution of 10 w/o ammonium fluoride and 10 w/o hydrogen peroxide inwater subsequent to the steps of etching and applying electrodes to aCZT detector crystal.

FIGS. 2A and 2B and FIGS. 3A and 3B, (where FIG. B in each caseillustrates a magnified view of FIG. A), shows the marked improvement inleakage current reduction of CZT materials processed by the presentinvention as is seen be the wide difference in slopes of thebefore-and-after current-voltage measurements. Actual device resistanceand leakage current, measured under an electrical bias of 100V, for lowresistance and high resistance CZT material is shown in TABLE 1.

It can be seen that treating the surface of a CZT crystal with asolution of 10 w/o NH₄F and 10 w/o hydrogen peroxide in water serves toreduce significantly the surface leakage current of the treated CZTcrystal. Providing a dielectric coating by oxidizing the surface ofetched CZT crystals substantially reduces surface leakage currents byreducing the effect of high conductivity residues, such as Te, left onthe etched surface at the completion of the etching process. Thisdielectric coating, grown directly from the CZT surface, allows for thedifference in bias voltage between adjacent electrodes, on strip orcoplanar grid detectors, to be increased without the onset ofcatastrophic noise effects thereby increasing the sensitivity and energyresolution of the detector. It is preferred that the ammoniumfluoride/hydrogen peroxide solution treatment time be between 5 to 10minutes.

In summary, the present invention provides a novel method for reducingthe leakage current of CZT crystals, particularly Cd_(1−x)Zn_(x)Tecrystals (where x is less or equal 0.5), and preferablyCd_(0.9)Zn_(0.1)Te crystals, thereby enhancing their ability tospectrally resolve radiological emissions from a wide variety ofradionuclides. The present method provides for etching the surface of aCZT crystal with a solution of bromine in methanol and subsequentlygrowing a dielectric layer on the crystal surface, preferably with aaqueous solution of ammonium fluoride and hydrogen peroxide.

From the foregoing description, one skilled in the art can readilyascertain the essential characteristics of the present invention. Theinventors, however, now wish to describe several modes for carrying outtheir invention and thereby providing a detector having improvedspectral resolution.

As illustrated in FIG. 4 the inventors show a conventional methodwherein exposed areas of a crystal detector are passivated after formingelectrical contacts. A contact pattern is placed onto the surface of aCZT crystal using standard mask-and-etch photolithographic techniques.Electrical contacts are then deposited onto the exposed surface usingany number of known techniques, including CVD, sputtering, and wetchemical methods such as electroless gold plating. Following attachmentof leads wires to the contacts the surfaces having metal electrodesdeposited on them are coated with a moisture barrier such as HumiSeal®.Remaining exposed surface of the crystal are subsequently passivated bygrowing a native dielectric/oxide layer thereon by any of the knowntechniques described above by the inventors or by the process of instantinvention.

Embodiment 1

Illustrated in FIG. 5A the inventors show a first mode for providing apassivated CZT crystal in which those exposed areas of a crystaldetector not already covered by electrical contacts, are passivated bythe ammonium fluoride/hydrogen peroxide solution treatment of theinstant invention. Again, a contact pattern is placed onto the surfaceof a CZT crystal using standard mask-and-etch photolithographictechniques. Metal contacts are then deposited onto the surface of theCZT crystal exposed by etching away an exposed portion of photoresist.Deposition means are again any suitable technique, including vapordeposition, sputtering, and wet chemical methods such as electrolessgold plating. Following formation of these electrical contacts theentire crystal is treated by the process of the present invention.

In a related manner, illustrated in FIG. 5B, an embodiment is shownwhich comprises forming the native oxide after a first mask-and-etchstep instead of forming the electrode contact. This step is thenfollowed by a second mask-and-etch step wherein a pattern is createdaround the oxide layer for the purpose of forming the contact.

The preferred mode for practicing embodiment 1 of the instant inventionis illustrated and shown in FIG. 6. A method is disclosed for providinga passivated CZT crystal wherein the passivating, dielectric/oxide layeris incorporated onto the CZT crystal before processing to provide thecrystal with electrical contacts. As with the mode illustrated in FIG.5, a contact pattern is placed down onto surfaces of the CZT crystalusing known mask and etch photolithographic techniques. In the preferredbest mode, however, the pattern is created into the native oxide filmalready grown on the surface of the crystal. This approach provides ameans for avoiding potential damage to electrical contacts or to theinterfacial zone beneath the contacts by the peroxide/ammonium fluoridepassivating solution of this invention.

Embodiment 2

A second embodiment comprising applying an encapsulating coating overthe previously passivated crystal surfaces.

It is known that various gases present in the atmosphere, especiallywater vapor, can have a deleterious effect on the long-term stability ofthe CZT crystal surface and passivating layer. In order to prevent or atleast minimize the chemical reaction of these species with the CZTcrystal surface. The inventors have discovered that the application of alayer of a polymer material known as HumiSeal® to the CZT crystalprovides an effective barrier to these gaseous species. The inventorshave found that an effective layer of this material may be applied bysimply dipping the passivated crystal into the liquid HumiSeal® polymerfor between 1 and 20 seconds after electrical leads have been attached.

The inventors have also noted in related, co-pending U.S. patentapplication Ser. No. 09/118,691, that by a depositing a hard-coatsilicon nitride layer onto the CZT crystal surface after that surfacehas been passivated and after electrode patterning and deposition asimilarly effective barrier is provided. That is, after oxidizing theCZT surface via the ammonium fluoride/hydrogen peroxide solutiontreatment of embodiment 1 a silicon nitride layer is applied whichfurther enhances the passivating capability of this technique. Thismethod, therefore, provides a novel process wherein a thin hard-coatoverlayer (nominally 1000 Å thick) of reactively-sputtered siliconnitride is laid down onto the passivated CZT surface. It is postulatedthat this overlayer prevents conduction induced by moisture or othergases in the operating environment. The conditions used for this processare shown below in Table 2.

TABLE 2 Deposition conditions for sputtered silicon nitride on CZT.Pressure N₂ flow Ar flow RF Power Target bias 5.5 mTorr 6 sccm 21 sccm100 Watts −300 Volts

These parameters were chosen to mimic those found in the prior art to beoptimal in terms of stoichiometry, refractive index, and residualstress. However, the properties of reactively sputtered films aresignificantly dependent on the characteristics of a particularsputtering system. Because the instant invention utilizes a systemdifferent from the prior art system in which the silicon nitride filmswere optimized, it is not clear to what extent the electrical propertiesof these particular silicon nitride films were similarly optimized. Itis thus highly probable that improved passivation with silicon nitridealone can be achieved with optimization of the silicon nitridedeposition process.

The sputter process itself is conventional. A diagrammatic sketch of theprocess is shown in FIG. 7. Radio frequency (RF) power supply 300 isattached to a standard magnetron sputter gun 301 which is itself heldwithin a high vacuum chamber 302 connected to a high speed, cryo-pumpvacuum pump 303. A target element 304 is held in the sputter gun inelectrical contact with the RF source 300. Argon and Nitrogen processgas is admitted into chamber 302 at which point the argon atoms 305 areionized by the RF field (not shown) and accelerated toward target 304.The high energy magnet in gun 301 retains the argon ion 305 in a loopingcirculation pattern 306 which brings them repeated into contact withtarget 304. As these high energy ions strike the surface of target 304,material is dislodged and ejected by a momentum transfer process. As thetarget surface is “cleaned” by the action of the argon plasma the addednitrogen gas reactions with this surface to form a nitride layer. It isthis layer which eroded by the plasma, dislodged as nitride fragments307 and ejected with enough energy to travel to the surface of thepassivated CZT crystal 308, attached to substrate holder 309, which isto be coated.

A hard-coat silicon nitride layer is thus deposited onto the passivatedand oxidized surface of the CZT crystal. Other dielectric nitride filmsknown to those skilled in the art of reactive sputtering processes canprovide surface passivation for CZT crystals. Potential candidates,beyond that of silicon nitride, include boron nitride, germaniumnitride, aluminum nitride and gallium nitride. Significant variation inthe parameters used for both the silicon nitride deposition steps mayalso yield processes which provide significant passivating capability.

In summary, the present invention provides a novel method for reducingthe leakage current of CZT crystals, particularly Cd_(1−x)Zn_(x)Te(where x is greater than or equal to zero and less than or equal 0.5),and preferably Cd_(0.9)Zn_(0.1)Te crystals. The present method providesfor depositing, via reactive sputtering, a silicon nitride hard-coatoverlayer which protects the surface of CZT crystals passivated by theammonium fluoride/hydrogen peroxide solution treatment of the presentinvention from moisture and other gases in the operating environment.

The reader will appreciate that the foregoing description is onlyintended to be illustrative of the present invention and is, therefore,not to be construed to limitation or restriction thereon, the inventionbeing delineated in the following claims.

What is claimed is:
 1. An ionizing radiation detector, comprising: aCdZnTe crystal having one or more surfaces; and a passivating dielectriclayer grown on said surfaces, said layer consisting essentially of CdO.2. The detector of claim 1, wherein the passivating layer is about atleast 250Å thick.
 3. The detector of claim 1, further including aninsulating means for providing a barrier between said dielectric layerand gases present in ambient air.
 4. The detector of claim 3, whereinsaid insulating means includes a polymer layer on said dielectric layer.5. The detector of claim 3, wherein said insulating means includes areactively sputtered hard-coat nitride layer deposited on saiddielectric layer.
 6. The detector of claim 5, wherein the hard-coatnitride layer is selected from the group consisting essentially ofsilicon nitride, boron nitride, germanium nitride, aluminum nitride, orgallium nitride.
 7. The detector of claim 1, wherein the presence of thepassivating dielectric layer reduces an initial measurement of surfacecurrent leakage by at least two orders of magnitude.